Download - Direct Reading Instruments
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Direct-ReadingGas Detection Instruments
Presented by
Michael D. Shaw
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DIRECT-READING
Means that instrument provides display
in engineering units (usually ppm or percent)
Implication (not always true) that unit is reading in real time
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GASES OF INTEREST
Combustible
Oxygen (deficiency and enrichment)
Toxic
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MOST COMMON TECHNOLOGIES EMPLOYED
IN INDUSTRIAL HYGIENE APPLICATIONS
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CATALYTIC SENSORS
Theory - Target gas is oxidized on catalytic element
The change in
temperature causes
a change in
resistance that is
measured by the
meter
Application combustible gases
(also known as Pellistors)
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CATALYTIC SENSORS
Advantage
Long life
Disadvantages
Different responses for each combustible gas
Can be poisoned
Most require at least 10% oxygen to work properly
Limited to percent-level detection
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METAL OXIDE SEMICONDUCTOR
(Also known as solid state, Figaro, or Taguchi sensors)
TheoryTarget gas reacts with MOS (SnO2 ) and
changes its resistance
as measured by the meter
Application - Nearly all oxidizable gases
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METAL OXIDE SEMICONDUCTOR
AdvantageInexpensive
Disadvantages
Not selective, and this is often misrepresented
Affected by humidity
Not truly analyticalbetter as go/no-go detectors
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NON-DISPERSIVE INFRARED (NDIR)
TheoryTarget gas absorbs infrared light at a particular wavelength
Via Beers Law
calculations
concentration
can be determined
Application Hundreds of compounds, especially organics
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BEERS LAW ( FOR EXTRA CREDIT )
"The deeper the glass,The darker the brew,The less of the incidentLight that gets through"
I = I0ekP
I = intensity of light striking the detector
I0 = measured signal at zero concentration of target gas
k = system dependent constant
P = concentration of target gas
e = base for natural logarithms
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NON-DISPERSIVE INFRARED (NDIR)
Advantage
Extremely versatile
Disadvantages
Expensive
Can be fragile and complicated to service
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PHOTOIONIZATION DETECTOR (PID)
Theory - Ultraviolet light ionizes the target gas
Current produced is proportional to concentration
ApplicationVolatile Organic Compounds (VOCs)
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PHOTOIONIZATION DETECTOR (PID)
Advantage
Detects a wide range of volatile organic compounds
Quite effective when used in conjunction
with chromatographic column Portable GCs
Disadvantages
Nonselective among organic vapors below
ionization potential of lamp
Affected by high humidity
UV lamps are expensive
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ELECTROCHEMICAL SENSORS
TheoryTarget gas is adsorbed on an electrocatalytic sensingelectrode, after passing through a diffusion medium, and is
electrochemically reacted.
The current producedby this reaction isdirectly proportional
to the gasconcentration.
Applications
Br2, CO, Cl2,ClO2, C2H4,
ethylene oxide
HCHO, H2, hydrazine, HBr, HCl, HCN, H2S, NO, NO2, O3,
propylene oxide, SO2, and oxygen-- among others
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ELECTROCHEMICAL SENSORS
Advantages
Inexpensive
Linear output
Can be miniaturized
Disadvantages
Prone to interferences in unskilled hands
Affected by temperaturereadily correctable
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WHAT IS THE MOST IMPORTANT THINGIN GAS DETECTION WHEN USINGDIRECT-READING INSTRUMENTS?
PROPER CALIBRATION!!
Without a clean zero gas and an accurate verified
calibration standardthere is no point in doing any gas
detection
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MONITORING PRODUCTS
OFFERED BY
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PORTABLE ANALYZER
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CONTINUOUS MONITORING SYSTEMSINGLE POINT APPLICATION
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CONTINUOUS MONITORING SYSTEMTWO POINT APPLICATION
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CONTINUOUS MONITORING SYSTEMMULTI-POINT (THREE OR MORE) APPLICATION
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ARC-MAX
DATA ACQUISITION, ARCHIVING,
AND REPORTING
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ARC-MAXMAIN SCREEN
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ARC-MAXSHIFT REPORT
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ARC-MAXTRENDING
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ARC-MAX ALARM LOG
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For Survey Applications,
We Recommend OurNomad Data Logger
12-bit resolution
Models available for
0-100mV, 0-2.5V
and 4-20mA inputs
Self-contained units
also available for
temperature, humidity
and pH
Sampling rate adjustable1 second to 10 hours in 1 second intervals
Storage capacity16336 readings
Excellent software included
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Visualize your measurement session with an auto-scaling time history graph
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Time history graph detail screen
Zoomed in on particular portion of the curve
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Ability To View All Individual Data Points
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CONCLUDING REMARKS
We've looked at the most widely used operating
principles in direct-reading gas detection instruments
We've touched upon the importance of calibration
We've examined two approaches to data acquisition
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SPECIAL THANKS TO
PROFESSOR GEORGE BYRNS